As the title says, after activating a weaponskill or spell that is part of a combo, the next stage of the combo will light up on the action bar, but the actual WS/spell used will be delayed in demonstrating that it was used and put on cooldown.
edit: just to clarify, this is a brand new 1.22a bug.
edit2:
ISP: Verizon
Type of Internet Connection: DSL
Internet Connection Speed: 3.5Mb/s
Date & Time: from 2012/05/09 (May 9th) 5 PM PDT to current
Frequency: Always
Character Name: Carraway Author
Race: Hyur M
World: Durandal
Class: BLM, MNK, DRG, WAR
Physical Level: 50
Area and Coordinates: Worldwide, instanced and non-instanced
Party or Solo: Party and solo
NPC Name: None
Monster Name: Garuda, Myrmidon Princess, Myrmidon Marshal, Myrmidon Soldier, Myrmidon Guard, Myrmidon Scavenger, Miser's Mistress, Chimera, Coincounter, Natalan Boneknitter, Natalan Gatekeeper, Watchwolf, etc.
Steps:
1. Activate the first step in any combo, including weaponskills and magic spells, i.e. Thunder
2. Watch action bar indicator for Thunder and Thundara: Thunder will not gray out and display cooldown immediately, but Thundara will light up as an active combo skill